Exposure, chemical duration, importance

No studies were located in humans or animals regarding the absorption of inhaled 1,1-dichloroethane. However, its use as a gaseous anesthetic agent in humans provides evidence of its absorption. Furthermore, the volatile and lipophilic nature of 1,1-dichloroethane favors pulmonary absorption. Structurally related chlorinated aliphatics and gaseous anesthetics are known to be rapidly and extensively absorbed from the lung. The total amount absorbed from the lungs will be directly proportional to the concentration in inspired air, the duration of exposure, the blood/air partition coefficient of 1,1-dichloroethane, its solubility in tissues, and the individual s ventilation rate and cardiac output. One of the most important factors controlling pulmonary absorption is the blood/air partition coefficient of the chemical. The concentration of the chemical and the duration of exposure are also important determinants of the extent of systemic absorption. 

When adequate data describing exposure concentration-duration relationships for a specific chemical and toxic endpoint of interest are not available, an alternative approach to estimating this relationship quantitatively must be followed. The approach used by the NAC/AEGL Committee involves the application of the equation C x t = k and the selection of a value or values of n that results in AEGL values that best fit the supporting data for the chemical and toxic endpoint in question. It is important to distinguish the difference between the derivation of values of n as described in the preceding section and the selection of values of n as described in this section. 

Determining anticipated route and magnitude of exposure is an important component in the overall assessment of safety and must be done on a nanomaterial-by-nanomaterial basis, with secondary exposures taken into consideration when necessary. The estimated exposure levels for a nanomaterial may then be compared with the calculated safe dose derived from the hazard identification evaluation. The procedures and factors considered in the exposure assessment process are not expected to be any different for nanomaterials than for larger particles or chemicals. The degree of hazard associated with exposure to any chemical or substance, regardless of its physicochemical characteristics, depends on several factors, including its toxicity, dose-response curve, concentration, route of exposure, duration and/or frequency of exposure. However, depending on the route of anticipated exposure (dermal, inhalation, oral) and types of associated toxicities (local or systemic), a chemical may not pose any risk of adverse effects if there is no... 

Biodegradation is one of the most important processes governing the environmental fate of a chemical.Hence, it is necessary to estimate which chemicals will be persistent or degradable either in treatment plants or in the natural ecosystems under aerobic and anaerobic conditions. The environmental concentration of a chemical, duration of exposure, soil type, water content, oxygen concentration, nutrients, temperature. 

The important factors, on exposure to chemicals that are toxic by absorption via the skin, are the contact area and the duration of exposure (refer to Table 11.17). 

However, the mathematics describes an idealized situation, and the real situation in vivo may not be so straightforward. For example, with carbon monoxide, as already indicated, the toxicity involves a reversible interaction with a receptor, the protein molecule hemoglobin (see chap. 7 for further details of this example). This interaction will certainly be proportional to the concentration of carbon monoxide in the red blood cell. However, in vivo about 50% occupancy or 50% carboxyhemoglobin may be sufficient for the final toxic effect, which is cellular hypoxia and lethality. Duration of exposure is also a factor here because hypoxic cell death is not an instantaneous response. This time-exposure index is also very important in considerations of chemical carcinogenesis. 

OPs have been in use for several decades as important chemicals for the control of crop pests. With their chemical and biochemical reactions, OPs have been well established as extremely poisonous chemicals. This classification is due to the inhibition of the marker enzyme ChE, which is produced in the liver. Blood enzymes provide an estimate of tissue enzyme activity. After acute exposure to OPs or a nerve agent, the erythrocyte enzyme activity most closely reflects the activity of the tissue enzyme. Once the OPs inhibit the tissue enzyme, it cannot hydrolyze ACh, and the accumulation stimulates the affected organ. Based on the manner of exposure (dose and duration) to different OPs, a series of toxicity signs and symptoms set in the organism, leading to death. These are important aspects to be closely monitored among pest control operators and occupational workers exposed to OPs. 

The most important factor is the dose-time relationship. The amount of a substance that enters or contacts a person is called a dose. An important consideration in evaluating a dose is body weight. Dose is the quantity of a chemical substance that a surface, plant, or animal is exposed to. Time means how often one is exposed to or the duration of exposure to a chemical substance. In simple terms, the dose-time relationship provides information on how much of the test substance is involved and how often the exposure to the test substance occurs. This relationship gives rise to two different types of toxicity of a chemical substance—namely, acute toxicity and chronic toxicity. 

This process measures the intensity, frequency, and duration of exposure to an agent. More precisely, it evaluates the potential exposure to each chemical of concern by the potential receptor populations or indicator species that are biotic components of any ecosystems in the site which may be affected. Two important components of an exposure assessment are (a) an exposure pathway analysis, and (b) the estimation of exposure point concentrations. To evaluate potential exposures, one requires concentrations of the chemicals of concern in the exposure media (e.g., surface water or sediments). 

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